Constant current device
j. h. reed



Jun 28, 1966 J. H. REED 5 Sheets-Sheet 1 M F/G. 5

A5 Has INVENTOR. JA MES H. REED BY r: A G Z ATTORNEY June 28, 1966 J. H. REED 3,258,679

CONSTANT CURRENT DEVICE Filed June 6, 1962 5 Sheets-Sheet 2 Q Q t La Q o a K) R INVENTOR.

JA MES H. 17550 ATTORNEY June 28, 1966 J. H. REED 3,258,679

CONSTANT CURRENT DEVICE Filed June 6, 1962 5 Sheets-Sheet 3 I INVENTOR.

JAMES h. REED ATTORNEY United States Patent 3,258,679 CONSTANT CURRENT DEVICE James H. Reed, Yerkes, Pa., assignor to Mauchly Assocrates, Inc., Fort Washington, Pa., 'a corporation of Pennsylvania Filed June 6, 1962, Ser. No. 200,566 3 Claims. (CL 323-19) In general, this invention relates to a new and improved constant current device utilizing static electronic components, and more particularly, to a constant current device capable of current regulation over a wide range which needs only a single controlled static electronic element for its operation.

Heretofore, constant current devices have been developed which had limited ranges of operation and slow response times. These constant current devices utilized magnetic amplifiers, transistors, and vacuum tubes but were all limited by the response of the device used and the capacitors and inductors included in the circuitry.

It is the general object of this invention to avoid and overcome the foregoing and other difficulties of the prior art by the provision of a better and more improved constant current device.

Another object is to provide a simple and better constant current device which is limited in response only by the response time of the controlled static element in its circuit.

Another object is to provide a new and improved constant current device having a wide range of operation.

Another object is to provide a new and improved constant current device whose current output and regulation can be varied by parameters of its circuitry.

Another object is to provide a new and improved constant current device utilizing 100% feedback.

Another object is the provision of a constant current device utilizing vacuum tubes which is operative at extremely low input voltages.

Another object is to provide a constant current device which may be utilized with any multi-grid vacuum tube.

Another object is to provide a constant current device whose upper voltage limit is determined by the electrical characteristic of its static controlled element.

Other objects will appear hereinafter.

For the purpose of illustrating the invention there is shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is a schematic showing of a standard pentode arrangement utilized to determine standard pentode curves.

FIGURE 2 is an embodiment showing one step in the development of the present invention.

FIGURE 3 is another embodiment showing still another step in the process of developing the present invention.

FIGURE 4 is a pentode arrangement utilizing the principles of the present invention.

FIGURE 5 is a triode arrangement using the principles of the present invention.

FIGURE 6 is a second embodiment of a pentode arrangement utilizing the principles of the present invention.

FIGURE 7 is a set of plate current versus plate voltage curves for the circuits shown in FIGURES 1-4.

FIGURE 8 is a set of curves taken at different curve values for the embodiment shown in FIGURE 4.

FIGURE 1 is a standard pentode arrangement utilized to deter-mine plate current versus plate voltage curves of the pentode.

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In this circuit, there is shown a pentode 10 having a plate 11, supressor grid 12, screen grid 13, control grid 14, and cathode 15. As is normal in this configuration, the suppressor grid 12 is connected directly to the cathode 15. The control grid 14 is connected to ground and the screen grid 13 is connected through a screen grid battery supply Egl to ground. The plate 11 is common with a tenrninal A1 and connected to a variable plate voltage supply Epl. The cathode 15 is connected through a resistance R1 to ground.

A plate current versus plate voltage curve was obtained for the circuit of FIGURE 1 and is shown as curve 1 in FIGURE 7. The following components were used in the circuit of FIGURE 1 to obtain the curve:

Tube 10 6AU6. Egl 112 volts. R1 2.2K ohms.

As can be seen from curve 1 of FIGURE 7, from 5 volts to 450 volts the current varied .65 milliampere. From 450 volts, varying Epl downwardly, a current swing of .02 milliampere occurred when Epl reached volts. Therefore, it can be said that the workable range for this configuration to obtain .02 milliampere swing was approximately 165 to 450 volts.

The first step in achieving a better constant current generator using a pentode was the placement of a constant voltage source between the screen grid and the cathode. This embodiment is shown in FIGURE 2.

In FIGURE 2, there is shown a pentode 20 having a plate 21, suppressor grid 22, screen grid 23, control grid 24, and cathode 25. The suppressor grid 22 is connected to the cathode 25. The control grid 24 is connected to ground and the plate 21, common with the output terminal A2, is connected through a variable plate voltage supply Ep2 to ground. A screen grid battery Eg2 is connected between the screen grid 23 and the cathode 25, thereby regulating the voltage between screen and cathode and maintaining it constant. The cathode 25 is also connected through a cathode resistor R2 to ground.

This embodiment improves the constant current characteristic of the pentode 20 in a manner shown with reference to curve 2 of FIGURE 7. To obtain curve 2, the following values were utilized:

Tube 20 6AU6. Eg2 112 volts. R2 3.3K ohms.

As can be seen with reference to curve 2 of FIGURE 7, the following current regulation was obtained by the improvement noted above. The plate current varied .18 milliampere from 5 volts to 450 volts. Corning down from 450 volts, it was necessary to lower battery Ep2 to 25 volts to obtain a .02 milliampere current swing.

A further improvement in the constant current characteristic of a pentode was accomplished by placing the re sistor from the screen grid to the control grid to provide a feedback circuit therebetween. In FIGURE 3, there is shown a pentode 30 having a plate 31, suppressor grid 32, screen grid 33, control grid 34, and cathode 35. The cathode 35 is connected directly to the suppressor grid 32. The plate 31 has a common terminal A3 to which is connected the positive terminal of a variable plate voltage battery Ep3. A screen grid battery Eg3 has its positive terminal connected to the screen grid 33 and its negative terminal to the cathode 35. A cathode resistor R3 is connected between the cathode 35 and the negative terminal B3 of the variable plate voltage supply Ep3. A screen grid resistor Rs3 is connected between the screen grid and the terminal B3. The terminal B3 is directly connected to the control grid 34.

The curve 3 shown in FIGURE 7 was obtained by 3 utilizing the circuit shown in FIGURE 3. To obtain the curve 3, the following values were utilized:

Tube 30 6AU6.

Eg3 112 volts. R3 6.8K ohms. Rs3 470K ohms.

In this embodiment, the current varied only .1 milliampere from volts to 450 volts plate voltage. A .02 milliampere swing was obtained between volts and 450 volts plate voltage. This was an improvement over curves 1 and 2 discussed with respect to FIGURES 1 and 2.

It was determined that by increasing resistor R3, an improvement could be had in the constant current characteristic of the embodiment shown in FIGURE 3. By increasing R3 to 10K ohms while reducting RS3 to 180K ohms, the following results were obtained. The range in which there was a .02 milliampere swing was then 12 volts to 450 volts. The current varied only .075 milliampere between 5 volts and 450 volts. This improvement was directly correlated to the increase in the resistance of resistor R3. The final step in the development of the present invention was the elimination of R3 entirely. By removing R3 from the circuit and adjusting Rs3 to obtain the desired operating current level, a great change in constant current regulation was obtained. This new embodiment is shown in FIGURE 4.

In FIGURE 4, there is shown a pentode 40 having a plate 41, suppressor grid 42, screen grid 43, control grid 44, and cathode 45. Suppressor grid 42 is directly connected to the cathode 45. The positive terminal of screen grid battery Eg4 is connected to the screen grid 43 and its negative terminal is connected to the cathode 45. The positive terminal of the screen grid battery Eg4 is also connected through a resistor Rs4 to the negative terminal B4 of the plate voltage battery Ep4. The control grid 44 is also connected to the negative terminal B4 of the plate voltage battery E124. The plate battery Ep4 has its positive terminal A4 connected to the plate 41 of the pentode 40. The curve 4 of FIGURE 7 was obtained utilizing the following components in the circuit of FIGURE 4:

Tube 40 6AU6. Eg4 112 volts. Rs4 120K ohms.

Between 5 volts and 450 volts plate voltage swing, there was a variation in plate current of approximately .005 milliampere. To obtain a swing in plate current of .02 rnilliampere, it was necessary to bring the plate voltage down from 450 volts to 2.5 volts. T his, as can readily be seen, was a tremendous improvement in the constant current characteristic of the pentode.

This extreme constant current regulation can be better understood with reference to the equations explaining the circuit of FIGURE 4.

( P=Vpk-Eg+IpRs Where Ep is the applied plate voltage, Vpk is the voltage drop between plate and cathode of the tube 40, Eg is the screen grid battery Eg4, Ip is the plate current, and Rs is the screen grid resistor Rs4.

(2) AEp=AVpk+AIpRs Since Eg is a constant voltage supply, there will be no change in its voltage and, therefore, this term of Equation 1 can be eliminated.

(3) AVpk= AVgk+AIpRp-askAVsk Where ,u is the amplification factor of the tube between control grid and cathode, AVgk is the change in control grid to cathode voltage, Rp is the plate resistance of the tube, ,usk is the amplification factor of the tube between screen and cathode, and AVsk is the change in voltage between screen and cathode. This formula and a derivation thereof is found on page 124, Equation 26 of the book Electronics by George F. Corcoran and Henry W. Price, published by John Wiley & Sons, Inc., 1954. Since AVsk remains zero in the embodiment shown in FIGURE 4, the term --;rskAVsk can be dropped from Equation 3.

(4) AVgk=AlpRs Substituting Formula 4 into Formula 3, we obtain (5) AVpk gAlpRs-kAlpRp Substituting Equation 5 into Equation 2, we obtain In a normal pentode, such as was shown in FIGURE 1, Alp equals AEp/Rp. Thus, the circuit of FIGURE 4 gives a far greater current regulation over the same voltage swing. For example, in the embodiment shown in FIG- URE 4, AEp was varied between and 1000 volts. The ,u of the 6AU6 is approximately 5000. The plate resistance Rp over this range of voltage varies between .5 and 1.5 megohms. For purpose of illustration, this will be taken as a constant value of 1 megohm. Substituting into Equation 7, for a change in Ip, we obtain the result that there is approximately 1.5 microamperes change in current as the voltage varies from 100 to 1000 volts. In fact, as the term [LRS becomes large as in the example given, the terms Rs+Rp in the Equation 7 become negligible.

The same theory of operation can be applied to a triode as is done in FIGURE 5. In this embodiment, there is shown a triode 50 having a plate 51, control grid 54, and cathode 55. A cathode voltage Eg5 is placed in series with a resistor Rs5 between the cathode 55 and the control grid 54. The plate voltage EpS is connected between the plate 51 and the control grid 54. Thus, the triode embodiment shown in FIGURE 5 also has high cathode impedance. If an analogy is taken to the equations discussed with reference to FIGURE 4, it will be seen that they apply equally well to the triode embodiment of FIGURE 5. g In fact, the final Equation 7 directed to the formula for changes in plate current is equally applicable to the triode embodiment. However, a triode is noted for its low values of ,u and, therefore, it is necessary to have an extremely large resistor RS5 to accomplish the desired current regulation.

It should be noted at this time that the battery generally designated by the terms Eg in FIGURES 2-4 is utilized to maintain the pentodes in a triode state with the screen grid acting as the plate of the triode. In this state, it is only necessary for enough voltage to be applied to the plate of the pentode to drive the control grid negative. Since this amount of voltage is usually quite small, the tube will start conducting as a constant current device at a much lower voltage than it is normally accustomed to operate at. The same principle is applied to the triode shown in FIGURE 5 as the battery supply Eg5 causes the control grid 54 and cathode 55 to act as a forward biased diode with the resistor Rs5 as a limit on the current in the circuit. Since it is already conducting as a diode, very little voltage is necessary between the plate 51 and control grid 54 to cause the triode 50 to reach its normal operating conditions. It can be seen that the invention could be designed for constant current operation from zero volts Ep by a simple voltage crossover network by one skilled in the art.

In FIGURE 6, there is shown another embodiment following the teachings discussed with respect to FIGURE 4. As was previously mentioned, the current regulation is dependent on the value of the resistor Rs4. Increasing the value of the resistor Rs4 increases the current regulation of the pentode. However, increasing the value of resistor Rs4 decreases the operating current level of the tube. Therefore, the embodiment shown in FIGURE 6 was developed to provide a circuit in which any operating current level may be obtained with any degree of current regulation.

In FIGURE 8, there is shown graphs of the circuit of FIGURE 4 at various values of Rs4. It can be seen that the degree of regulation increased as the values of Rs4 increased. However, it is also quite obvious that increases in the value of Rs4 decreased the current level of the circuit.

FIGURE 6 has eliminated this problem by the addition of a battery in series with the screen grid resistor. In FIGURE 6, there is shown a pentode 60 having a plate 61, suppressor grid 62, screen grid 63, control grid 64, and cathode 65. The suppressor grid 62 is connected directly to the cathode 65. The screen grid 63 is connected to the positive terminal of a battery Eg6 whose negative terminal is connected to the cathode 65. The positive terminal of the battery Eg6 is also connected to the negative terminal of a screen grid battery E06. The positive terminal of the battery B06 is connected through a screen grid resistor Rs6 to the control grid 64. The control grid 64 has a common terminal B6. The plate 61 has a common terminal A6 and the plate battery Ep6 is connected between the terminals A6 and B6. To increase the value of resistor Rs6 is to increase the current regulation of the circuit. The value of battery E06 has no effect on the current regulation characteristic of the circuit. However, the sum of the voltages of batteries Eg6 and E06 along with Rs6 determines the operating current level of the tube 60. Since it is not possible to increase the voltage of battery Eg6 over a maximum amount determined by the electrical limitations on screen grid voltage, the battery E06 is utilized to provide the necessary increases in operating current level.

Many variations of the circuit in the present invention can be obtained by changes in the components utilized. For this purpose, solid state devices such as transistors could be readily substituted for the vacuum tube components discussed previously. For this reason, the cathode should be generally noted as an electron emitter, the anode as an electron acceptor, the control grid as an electron controller, and the screen grid as an electron shield.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. A constant current device comprising an electronic discharge device having a cathode, plate, control grid and a screen grid between said control grid and said anode; means for connecting a variable voltage source between said cathode and plate; and a resistive degenerative feedback loop connected to supply a voltage signal to said control grid proportional to the current flowing through said electronic discharge device due to changes in the voltage of said variable voltage source, a high resistance connected between said cathode and said control grid, and said resistive feedback loop connected between said screen and said control grid.

2. The constant current device of claim 1 including a source of constant voltage connected between said screen grid and said cathode.

3. The constant current device of claim 2 including a second selectively variable source of constant voltage in said resistive degenerative feedback loop, said second source having a polarity additive with said source of constant voltage connected between said cathode and said screen grid, whereby the output current of said electronic discharge device may be maintained constant by increasing the voltage of said second constant voltage source for an increase in the resistance of said resistive feedback loop.

References Cited by the Examiner UNITED STATES PATENTS 2,171,614 9/1939 Wendt 323-4 2,326,614 8/1943 Bowman 330-70 2,970,278 1/1961 Reaves 330-202 X JOHN F. COUCH, Primary Examiner.

LLOYD McCOLLUM, K. W. HADLAND, H. B. KATZ,

K. D. MOORE, Assistant Examiners. 

1. A CONSTANT CURRENT DEVICE COMPRISING AN ELECTRONIC DISCHARGE DEVICE HAVING A CATHODE, PLATE, CONTROL GRID AND A SCREEN GRID BETWEEN SAID CONTROL GRID AND SAID ANODE; MEANS FOR CONNECTING A VARIABLE VOLTAGE SOURCE BETWEEN SAID CATHODE AND PLATE; AND A RESISTIVE DEGENERATIVE FEEDBACK LOOP CONNECTED TO SUPPLY A VOLTAGE SIGNAL TO SAID CONTROL GRID PROPORTIONAL TO THE CURRENT FLOWING THROUGH SAID ELECTRONIC DISCHARGE DEVICE DUE TO CHANGES IN THE VOLTAGE OF SAID VARIABLE VOLTAGE SOURE, A HIGH RESISTANCE CON- 